1. Field of the Invention
The present invention relates to a plurality of secondary batteries connected in series, a charge/discharge control circuit equipped with an intermediate terminal disconnection detecting circuit for detecting disconnection with the secondary batteries, and a battery device.
2. Background Art
In a battery device equipped with a plurality of secondary batteries connected in series, when disconnection (hereinafter called intermediate terminal disconnection) of a connecting portion of each secondary battery and a charge/discharge control circuit occurs, it is not possible to perform charge/discharge control of the secondary batteries. That is, there was a problem that though the secondary batteries were in overcharge or overdischarge, it could not be detected, thus giving large stress to the secondary batteries. Therefore, there has been proposed a battery device equipped with an intermediate terminal disconnection detecting circuit for the purpose of detecting the intermediate terminal disconnection to thereby enhance safety of the secondary batteries.
A circuit diagram of a battery device equipped with a related art intermediate terminal disconnection detecting circuit is shown in FIG. 3. The battery device includes secondary batteries 107a and 107b, a charge control FET 106, a discharge control FET 105, external terminals 112 and 113, and a charge/discharge control circuit 201.
The charge/discharge control circuit 201 is composed of a positive power supply connection terminal 108 to which a positive power supply of the secondary battery 107a is connected, an intermediate terminal 109 connected to a connecting point of a negative power supply of the secondary battery 107a and a positive power supply of the secondary battery 107b, a negative power supply connection terminal 110 to which a negative power supply of the secondary battery 107b is connected, a voltage detection circuit 102 for monitoring the voltage between the positive power supply connection terminal 108 and the intermediate terminal 109, a voltage detection circuit 103 for monitoring the voltage between the intermediate terminal 109 and the negative power supply connection terminal 110, a control circuit 204 for outputting signals for controlling the charge control FET 106 and the discharge control FET 105 in response to the outputs of the voltage detection circuits 102 and 103, and an intermediate terminal disconnection detecting circuit 211.
The operation of the related art battery device will next be described.
When a voltage generator such as a charger is connected between the external terminals 112 and 113 to charge the secondary batteries 107a and 107b, the voltage detection circuit 102 outputs a detection signal to the control circuit 204 when the voltage of the secondary battery 107a reaches a predetermined voltage or higher set by the voltage detection circuit 102. Then, the control circuit 204 having received the signal therein outputs a signal for turning off the charge control FET 106 to turn off the charge control FET 106 to thereby avoid a charging current from flowing and thereby controls the secondary battery 107a so as to prevent its overcharging.
When a load such as a resistor is connected between the external terminals 112 and 113 to discharge the secondary batteries 107a and 107b, the voltage detection circuit 102 outputs a detection signal to the control circuit 204 when the voltage of the secondary battery 107a becomes a different predetermined voltage or less set by the voltage detection circuit 102. Then, the control circuit 204 having received the signal therein outputs a signal for turning off the discharge control FET 105 to turn off the discharge control FET 105 to thereby avoid a discharging current from flowing and thereby controls the secondary battery 107a so as to prevent its overdischarging.
The intermediate terminal disconnection detecting circuit 211 is set to supply more current than the current flowing through the voltage detection circuit 102. When the connection between the intermediate terminal 109 and the secondary battery 107 is disconnected, the voltage of the intermediate terminal 109 is pulled up by the intermediate terminal disconnection detecting circuit 211. Accordingly, the difference in voltage between the intermediate terminal 109 and the negative power supply connection terminal 110 becomes large. When it becomes a predetermined voltage or higher set by the voltage detection circuit 103, the voltage detection circuit 103 detects an intermediate terminal disconnection and outputs its signal to the control circuit 204. Since the intermediate terminal disconnection can be detected by providing the intermediate terminal disconnection detecting circuit 211 in this manner, it is possible to provide a high safety battery device (refer to, for example, FIG. 1 in Japanese Unexamined Patent Application Publication No. Hei 8(1996)-308115).
In the conventional charge/discharge control circuit and battery device, however, a difference occurs between current consumed by the secondary battery 107a and current consumed by the secondary battery 107b due to the current flowing through the intermediate terminal disconnection detecting circuit 211. A problem arose in that since this current difference became an unbalance current for each secondary battery to be connected and caused the balance between battery voltages to collapse, the service life of the battery device would be shortened.